Future batteries need to triple capacity, cut price by 67%

Stuffing lithium into a material causes it to expand; can we control it?

Battery research is one of the hottest areas of materials science, with a steady stream of promising ideas emerging from research labs. But even though battery performance has steadily climbed, a lot of that progress is due to an evolution of existing technology rather than an adoption of more radical ideas floating around in labs.

At the recent meeting of the American Association for the Advancement of Science, two of the people who run some of these labs gave good descriptions of why it has been so difficult to translate promising results into revolutionary products.

More capacity, lower price

Stanford's Yi Cui showed a slide that laid out the goals of battery research very simply. Right now, batteries cost about $300 per each kiloWatt-hour of capacity. For the two largest use cases (electric vehicles and on-grid storage), we need that figure to drop to about $100 per kW-hr in order for the technology to compete with fossil-fuel-powered cars and generating facilities. For the grid, where the batteries are stationary, it doesn't matter how much they weigh. But for a more effective electric vehicle, we'd like to see the energy density rise from its present 200 W-hr/kg to about 600 W-hr/kg.

That's tripling the capacity while cutting the price by two-thirds. A pretty tall order.

It's this challenge that's motivating Cui and fellow speaker Linda Nazar to look into new materials for battery electrodes.

Electrodes play a key role in batteries in that they're where charge carriers—lithium in today's batteries—are held. Their ability to store lithium therefore becomes a key determinant of the storage density of a battery. Right now, carbon electrodes require six atoms of carbon for each lithium atom stored. Elements further down that column in the periodic table, like silicon and germanium, however, have a more complicated electronic structure, which can interact with more lithium atoms. As a result, you can store 4.4 lithium atoms for each silicon atom—a significant boost in capacity.

So why aren't we using silicon in batteries already? The problem is that the added lithium atoms cause silicon to expand, damaging the integrity of the material. Cui's talk was essentially a history of his lab's attempts to overcome this problem.

His first idea was simply to use silicon nanowires; by adjusting the spacing, you're able to allow room for the expansion that occurs. Unfortunately, after a few charge/discharge cycles, the neat wires had become amorphous, gradually losing their structure integrities. From there, his lab tried placing a solid wire inside an amorphous silicon shell (which didn't work as well as hoped) and then placing the amorphous silicon on the inside of a hard tube, with the tube limiting the expansion of the lithium storage material. As an alternative, his lab also tried making hard spherical coats with amorphous silicon inside.

In these latter cases, the shell simply wasn't up to the task; Cui showed time-lapse imagery in which the shell stretched and then cracked under the strain.

At this point, a bit of help came in the form of a lab down the hall, which was working on self-healing polymers. These materials rearrange hydrogen bond networks over time, potentially restoring breaks. Cui's group coated amorphous silicon beads with this polymer mixed with conducting carbon to move charges in and out. If the coating breaks badly during a charge-discharge cycle, the polymer will gradually heal over the ruptures. "While you're sleeping, your battery can self-heal," Cui said.

Cui's formed a company that's commercializing his group's advances; right now, it has a 270 W-hr/kg battery on the market, and there is a 360 W-hr/kg version working in the lab. But he neglected to tell the audience which of the above technologies (if any) these batteries relied on.

Cheap and yellow

If we're still a bit short of the 600 W-hr/kg goal, Cui suggested he is still aiming high. His lab has started looking into lithium-sulfur batteries, which have a theoretical capacity of 2,500 W-hr/kg. Cui is simply seeing if he can form similar shelled structures, akin to the ones he used with silicon, that can get sulfur to behave better inside batteries. But he left it to the next speaker, Linda Nazar, to fill in the details.

Rather than storing ions, lithium-sulfur batteries undergo chemical reactions at an electrode, harvesting the electrons released in the process. Lithium can react with sulfur to form Li2S, but Nazar said that the reaction isn't quite this simple. Sulfur's native form is an eight-atom ring, and the reactions that break this down lead to intermediates called polysulfides that can leak away from the electrode and undergo reactions elsewhere in the battery.

To understand this process better, Nazar is constructing simplified batteries that are then probed with an X-ray beam generated by a particle accelerator. This process provides a spectrum of the reaction products at 15-minute intervals—"we know precisely what sulfur is up to," as she put it. The challenge for her group is getting the polysulfides to react before they can drift off.

X-raying batteries

One of the speakers at the session came from SLAC, the former particle accelerator converted into an intense X-ray laser. He talked a bit about technology that is changing our understanding of what happens in batteries. Researchers can now create simple batteries in a pouch or coin-shaped container that is transparent to X-rays. These batteries can be charged and discharged while being scanned with the high-intensity light generated by particle accelerators.

The X-rays can accomplish three things. One is simply imaging the battery material itself at a 30nm resolution, which helps researchers track any structural changes that occur during charge/discharge cycles. A second is spectroscopy, which identifies all of the chemical species present at specific points in the cycle. Finally, the X-rays can be used for crystallography, so we can understand how the atoms of the battery material are arranged.

This approach is only a few years old, but it has already shown up in a number of papers and featured in the talks of both Cui and Nazar.

Like Cui, her first approach was to simply try to find something that could contain them. After trying silicon- and titanium-dioxide shells, then switching to vanadium oxide, Nazar's team found that increasing the conductivity of the shell caused the sulfur reactions to go faster, allowing the reaction to proceed to completion before the polysulfides could escape. She eventually settled on Ti4O7, where the speed of electron transport cut polysulfide production in half.

That is a much better result, but it's still not ideal. So her group started taking a somewhat different approach: getting the polysulfides to react with the electrode itself, keeping them in place. She started with extremely thin, parallel aluminum sheets, which attract the reaction intermediaries to the surface. These sheets cut polysulfide release to the point where capacity only fades by 0.05 percent each charge/discharge cycle—similar, Nazar said, to the loss of performance in today's lithium batteries.

Her group is also working with a mineral called Bernecite, which naturally forms nanoscale sheets that sulfur readily interacts with, forming chains of thiosulfates on the surface of the sheets. In part, this interaction limits the polysulfides that escape the electrode. But even those that escape into solution continue to react at the electrode, which clears them out of the battery at full discharge.

Nazar is excited about getting sulfur to work in part because it's so cheap, at $170 a ton. At that price, it could help make storing renewable energy in batteries economically viable.

Overall, the researchers provided a perspective that's hard to get by following the flow of research papers. While many of these papers do represent significant advances, they're often partial solutions to the issues they're tackling or create new issues entirely. Many of the papers aren't intended to show up in a new battery; instead, they're meant to point us toward the materials that can eventually be commercialized.

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